Myoclonus and a number of other rapid involuntary movements have been difficult to classify clinically. Consolidation of patient material obtained outside of NIH with continuing analysis of new cases at NIH has led to new classifications and pathophysiological insights. Conclusions have been drawn about myoclonus of epileptic nature, adult onset tic and startle. Tremors are also difficult to classify. In studying the physiological aspects of these disorders, we have identified a new descriptor of cerebellar postural tremor which may help distinguish it from other postural action tremors. The amplitude of cerebellar postural tremor appears to depend on the precise posture of the limbs, and, in particular, is highest when the arms are near the body and the hands are pointing toward each other. The physiological implications of this are being explored. We have also studied several patients with orthostatic tremor, a curious, newly described disorder of tremor of the legs only when standing. Our preliminary studies suggest that this is a disorder of the balance mechanism. We plan to continue these studies and add PET scanning as a physiological tool to our battery of clinical neurophysiological methods to help identify abnormal areas in the brains of patients with involuntary movements.